The present invention relates to a stay which contains at least one pressure medium, suitable for use with a cover or the like, particularly a cover associated with the front or rear compartment of a motor vehicle, such as the hood of the engine compartment, be it located at the front or rear, or the cover of the trunk compartment, be it at the front or rear.
There exist stays of the above type which comprise a cylinder having within it a reciprocatingly mounted piston rod which is equipped with a piston having at least one opening through which the pressure medium may flow. One such stay, if, when filled with a gaseous medium, is known as an aircushion-type spring, is shown, for example, in German Utility Model (Gebrauchsmuster) No. 7,410,489, published July 17th, 1975. By filling the stay at least partly with a gaseous medium, the remainder of the pressure fluid medium being a hydraulic, i.e., liquid, medium, has the advantage that when the stay is connected to the cover or the like, the gaseous medium which on that side of the piston that faces away from the piston rod exerts a pressure on the piston which tends to move the piston rod out of the cylinder and thus at least partly helps to open the cover or lid. Using a liquid medium in addition to the gaseous medium, has the advantage that the arrangement as a whole is more easily sealed off from the outside.
A stay of the above type has to be so constructed and arranged that the braking action exerted by the stay is directionally selective. That is to say, if the stay is used, for instance, in connection with the rear cover of a motor vehicle, the arrangement of the parts should be such that the stay should not hinder the opening of the cover, on the contrary, it should facilitate it, while the parts should additionally be so arranged as to allow the cover to be held in its open position, and if possible in intermediate positions. On the other hand, the holding force which maintains the cover in its open position should not be so large as to lock the cover into its open position, instead, the stay should make it possible for the cover to be closed more or less easily through the application of a manual force. Indeed, the force required to close the cover should preferably be no greater than that which can be exerted by one hand, so that the person closing the cover can have the second hand free for carrying things or for any other purpose.
One way to obtain the directional selectivity would be by equipping the piston with different openings which are provided with respective check valves which act in opposite directions and respond to different pressures. Such an arrangement would in practice, be likely to have a number of disadvantages because the openings which allow the flow of the pressure medium through the piston must, if the cylinder is filled with a liquid medium, be sufficiently large to allow for the increased viscosity during cold-weather operation. This, in turn, makes it necessary to make the piston relatively large in order to accommodate such large size openings each of which, it will be remembered, serves to permit the flow of liquid in but one direction. Moreover, as the increased pressure which prevails on that side of the piston which is directed away from the piston rod and which is supposed to assist the opening of the cover has to flow through a check valve, any delay in the opening of this check valve will undermine whatever assistance the increased pressure might otherwise be able to provide.
It is, therefore, the primary object of the present invention to provide a stay for use with a cover or the like which overcomes the above drawbacks, which is of simple, sturdy and compact construction, and which is able to give long periods of trouble-free service.